If you are using a Mac, without going through the steps of using Bootcamp, will the MacOS see the Windows install on an external drive and offer it as choice to boot from. If that works (which I am questioning) you'd still have to install the apple drivers.
I've had this setup running for 2 years on a mac pro, there are reliable detailed guides for this (I could find the one I've used if you need it). There's absolutely no involvement of macOS in the procedure, so there's no need for macOS to "see" any drive. Mac's EFI will be able to boot from it as long as your external disk is bootable.
Regarding drivers, you don't really need apple's slacking-updated drivers, but you can still use them if you prefer them (you can just download them on a usb and install them - bootcamp or not doesn't matter for this, anyway). However, I used to install the latest AMD drivers directly from AMD site.